1. Field of the Invention
The present invention relates to a high pressure casting method such as, for example, a die casting method, for making an article of manufacture having a shaped hollow defined therein, and a casting core used in the practice of the high pressure casting method and having a shape complemental to the shape of the hollow in the article of manufacture.
2. Description of the Prior Art
It is well know that, in the practice of the high pressure casting method, for example, the die casting method for making an article of manufacture having a shaped hollow defined therein, a casting core complemental in shape to the shape of the hollow in the article of manufacture is used. The casting core is generally in the form of a compressed sand block that can be disintegrated, after the article of manufacture has been molded, so as to leave the complemental hollow inside the resultant article of manufacture. Numerous methods of making the casting core are well known, and, where during the practice of the die casting method a high pressure is used to inject molten metal, for example, molten aluminum, into a mold assembly, core sands are integrated together by the use of a core binder to complete the casting core. An example of the core making methods is disclosed in the Japanese Patent Publication No. 60-15418, published Apr. 19, 1985.
In general, during the casting of the article of manufacture having the hollow defined therein (which hollow is hereinafter referred to as a cored-out hollow, the article of manufacture having the cored-out hollow being referred to as a hollow article, for the sake of brevity) with the use of the sand core supported inside the mold assembly, molten metal injected into the mold assembly under pressure fills up a gap between the sand core and the wall of the mold assembly defining a mold cavity, contacting the outer surface of the sand core. It has often been experienced that sand inclusion occurs in the wall of the hollow article defining the cored-out hollow as a result of sand particles sticking to the molten metal then solidified. The inclusion of sand particles in the wall of the hollow article defining the cored-out hollow poses numerous problems. By way of example, where the hollow article is an automobile engine cylinder head, the sand particles would contaminate lubricant oil and/or a cooling water circulated in the cylinder head.
In view of the foregoing, the sand core disclosed in the previously mentioned Japanese publication has inner and outer coatings applied over the outer surface of the sand core. The inner coating is formed by applying and drying an aqueous slurry of powdery refractory material, for example, silica flour, zircon flour or mullanite flour, and colloidal silica containing 20 to 30% silica. The outer coating subsequently formed over the inner coating is formed by applying and drying and aqueous solution of artificial mica containing sodium, fluorine and silicon with or without lithium and capable of exhibiting fluidity when in contact with water. The aqueous slurry for the inner coating is of a composition containing 50 liters of water, 5 kg of colloidal silica containing 30% silica, 0.5 kg of a wetting agent (sodium dialkylsulfonesuccinate), 0.1 kg of a defoaming agent, 100 kg of zircon flour of 10 micron in particle size and 40 kg of zircon flour of 1 micron in particle size.
The aqueous solution for the outer coating is of a composition containing 15 liters of water, 30 kg of an aqueous solution of 15% artificial mica, 0.9 kg of the same wetting agent and 0.12 kg of a defoaming agent, or of a composition containing 30 liters of water, 30 kg of the aqueous solution of the artificial mica, 0.9 kg of the wetting agent, 0.12 kg of the defoaming agent and 3 kg of metallic aluminum powder.
According to the above mentioned Japanese publication, the inner coating is utilized to impart a smoothness to the surface of the wall defining the cored-out hollow in the resultant hollow article while, during the casting, acting to distribute the pressure of the molten metal being injected substantially uniformly over the surface of the sand core. The outer coating is utilized to prevent the molten metal under pressure from penetrating into interstices in the sand core during the casting.
It is, however, been found that, since particles of the mica used as a principal component of the outer coating do not exhibit a satisfactory dispersibility in the aqueous solution, the mica particles cannot be distributed uniformly in the outer coating. Moreover, the particle size of the mica is relatively great so much as to result in drooping of the coating material, constituting an additional cause of the uneven distribution of the mica particles in the outer coating.
The uneven distribution of the mica particles in the outer coating would result in uneven shrinkage of the outer coating during the subsequent drying process, which in turn constitutes a cause of occurrence of crackings in the outer coating. As a matter of course, the presence of crackings in the outer coating of the sand core permits molten metal to penetrate internally of the sand core during the casting process.
Even though the resin-coated sand core is successfully prepared with no cracking occurring in the outer coating, the outer coating tends to be susceptible to cracking during the casting process by the effect of a high pressure under which molten metal is poured into the mold cavity. In particular, where the resin-coated sand core has recesses in its shape, the outer coating is sensitive to shrinkage crack and, therefore, when such a resin-coated sand core is used in the practice of the high pressure casting technique, the frequency of occurrence of cracks in the outer coating tends to increase.
Moreover, when the resin-coated sand core is removed subsequent to the casting which has been carried out under these circumstances, there is a relatively high possibility that particles of one or both of the mica and the sand material forming the core may remain sticking to the wall defining the cored-hollow in the hollow article. The practical use of the hollow article so prepared as, for example, an automobile engine cylinder head poses a problem the use of the sand core in which no mica coating as a penetration preventing layer is formed, the sand and/or mica particles would contaminate a lubricant oil and/or a cooling water circulated in the cylinder head.